Hair straightening treatment has hitherto been known as a cosmetic technique for fixing curly or frizzy hair in a substantially straight state. Various small improvements have been made for hair straightening treatment, and they are generally done in the following manner. A reducing first agent of thioglycolic acid or the like serving as a reducing agent for cystine bonds that crosslink the main chains of keratin constituting hair is applied to the hair, and the hair is left for a certain time, thereby to break the cystine bonds, followed by washing with water and drying. By breaking the cystine bonds, the distorted cross-linked structure, which is the cause of curly or frizzy hair, is eliminated. Then, straight ironing is performed on the hair in which the cystine bonds have been broken, thereby shaping the hair into straight hair. Next, an oxidizing second agent containing an oxidizing agent such as hydrogen peroxide or sodium bromate is applied to the hair that has been shaped into straight hair, and the hair is left for a certain time, thereby to reform the cystine bonds, followed by washing with water and drying.
When the above-described reducing first agent is applied to the hair, the cystine bonds are broken to loosen the bonding between keratin molecules, resulting in a problem that the hair coloring dye attached to or charged in a matrix inside the hair is flows out to cause a color loss. For this reason, when hair straightening is successively performed immediately after hair dyeing, the hair coloring dye used for dyeing performed immediately before hair straightening flows out, so that it has not been possible to sufficiently retain the color of the dyed hair after hair straightening. Accordingly, to perform hair dyeing and hair straightening, it has been usually necessary to perform hair dyeing treatment after an interval of at least about one week after hair straightening treatment until the cystine bonds are sufficiently reformed. However, in such a case, a person to be treated needs to take time and trouble to make a visit to a hair salon twice on separate days for receiving hair straightening and hair dyeing. To save such time and trouble, there is a need for a method that enables hair dyeing and hair straightening to be performed by a single visit to a hair salon.
As a technique for solving such a problem, PTL 1 below discloses a method for simultaneously performing hair straightening treatment and hair dyeing treatment, which is a method for successively performing hair straightening treatment and hair dyeing treatment on hair at once by using a hair straightening agent composed of a hair reducing first agent and an oxidizing second agent, the method including a series of successive steps of: treating hair with the first agent into a reduced state; subsequently performing hair dyeing by using a treatment agent containing a basic dye and/or an HC dye; and oxidizing the dyed hair in a reduced state by treating the hair with the second agent.